Heretofore, a small-sized and thin model image pickup apparatus has been mounted on a mobile terminal such as a cell-phone and PDA (Personal Digital Assistant), and thereby, mutual transmission of not only sound information but also image information to remote places has become possible. As an image pickup element used for the image pickup apparatus, there are used a CCD (Charge Coupled Device) type and an image pickup element of a CMOS (Complementary Metal-Oxide Semiconductor) type.
With the recent increasing spread of these mobile terminals, there are now supplied, to the market, the mobile terminals each carrying an image pickup apparatus employing an image pickup element having a large number of pixels to obtain images with higher image quality. Some of these image pickup apparatuses employ an image pickup lens that is composed of a plurality of lenses for the purpose of an improvement of resolution, corresponding to image pickup elements having a large number of pixels. As an image pickup lens corresponding to an image pickup apparatuses having a large number of pixels, there has been proposed an image pickup lens composed of four lenses which can provide higher performance than a lens composed of two or three lenses.
As the image pickup lens composed of four lenses, Japanese Patent Publication Open to Public Inspection (JP-A) No. 2004-341013 discloses a so-called inverted Ernostar type image pickup lens. The inverted Ernostar type image pickup lens is composed, in the order from the photographic subject side, of the first lens having a positive refractive power, the second lens having a negative refractive power, the third lens having a positive refractive power, and the fourth lens having a positive refractive power, to achieve higher performance. Further, each of JP-A 2002-365529, 2002-365530, and 2002-365531 discloses an image pickup lens of a so-called telephoto type. The image pickup lens of the telephoto type is composed, in the order from the photographic subject side, of the first lens having a positive refractive power, the second lens having a negative refractive power, the third lens having a positive refractive power, and the fourth lens having a negative refractive power, to achieve a smaller size in terms of the total length of the image pickup lens.
However, the image pickup lens described in JP-A 2004-341013 has a positive fourth lens due to the inverted Ernostar type which provides a principal point position of the optical system closer to the image side and a longer back focus, compared with an image pickup lens of telephoto type which provides a negative fourth lens. Therefore, such type of image pickup lens as described in JP-A 2004-341013 is disadvantageous for downsizing. In addition, there is provided only one lens having negative refractive power among four lenses in the image pickup lens. Thus, it provides a difficulty in correction of Petzval sum, and in securing excellent performance on the periphery portion of the image area. The image pickup lens described in each of JP-A 2002-365529, 2002-365530, and 2002-365531 has a narrower imaging field angle and insufficiently corrects its aberrations. If the total length of the image pickup lens is further shortened, it becomes difficult to be used with an image pickup element with a large number of pixels due to deterioration of its performance.